Tandem accelerators are used to accelerate ions toward a target, often a semiconductor workpiece. A tandem accelerator is so named because it accelerates an ion beam two times. The tandem accelerator includes a plurality of input electrodes biased at progressively more positive voltages and a plurality of output electrodes biased at progressively less positive voltages, with a positively biased terminal disposed between these two pluralities of electrodes.
To create an accelerated ion beam, a negative ion beam is accelerated toward the positive terminal. This negative ion beam is stripped of electrons in the terminal to become a positive ion beam. The positive ion beam is then accelerated away from the positive terminal toward the less positive output electrodes.
To strip the negative ion beam of its electrons, the negative ion beam enters a stripper tube. Neutral molecules are also fed into the stripper tube. As the negative ions pass through the neutral molecules, electrons are stripped from the negative ions, transforming them into positive ions. These positive ions are then attracted from the positive terminal toward the more negative output electrodes.
Ideally, only positive ions that were created from the original negative ions pass through the tandem accelerators. However, in some cases, other ions, such as positive ions from the neutral molecules, also pass through the tandem accelerator. These undesired ions may contaminate the workpiece and decrease the yield of the implanting process.
Therefore, it would be beneficial if there were an improved tandem accelerator for creating high energy ion beams without introducing unwanted ions.